Recently, electrospinning (charge induction spinning) has been receiving attention, since it enables easy production of nanofibers, which are fibrous material having submicron scale diameters. According to electrospinning, a raw material liquid comprising a polymer material dispersed or dissolved in a solvent is extruded into the air. When a high voltage is applied to the raw material liquid to extrude it, the raw material liquid becomes electrically charged, and the raw material liquid is electrically drawn in the air to form nanofibers (see, for example, PTL 1).
More specifically, while the raw material liquid, electrically charged by the electric field and extruded into the air, is traveling in the air, the solvent evaporates and the volume of the raw material liquid decreases. However, the electrical charge of the raw material liquid is retained despite the evaporation of the solvent. Thus, the charge density of the raw material liquid increases as the solvent evaporates. When the repulsive Coulomb force in the raw material liquid overcomes the surface tension of the raw material liquid, the raw material liquid is explosively drawn linearly (hereinafter referred to as electrostatic drawing). Such electrostatic drawing occurs continuously in the air, and the raw material liquid is subdivided geometrically, thereby resulting in formation of microfibers with submicron scale diameters.
Also, PTL 2 proposes an apparatus for producing nanofibers by electrospinning, in which a raw material liquid is extruded from a rotatable container. As illustrated in FIG. 9, this apparatus includes: a spray head 102 having at least one extrusion element 101 in the peripheral wall; and a cylindrical collecting member 103 containing the spray head 102. A voltage is applied between the spray head 102 and the collecting member 103 by a high voltage power source 104 to generate an electric field therebetween. In this state, the spray head 102 is rotated. As a result, a raw material liquid 106 supplied into the spray head 102 through a passage 105 is extracted from the tips of the extrusion elements 101 by the electric field to produce nanofibers. The produced nanofibers are deposited and collected on the inner surface of the collecting member 103.
Also, PTL 3 proposes a technique in which a cylindrical container having a large number of orifices in the peripheral wall is rotated to extrude a raw material liquid for forming nanofibers from the orifices by centrifugal force. In PTL 3, as illustrated in FIG. 10, a raw material liquid 114 for forming nanofibers is supplied to a cylindrical container 111 having a large number of orifices 113 in the peripheral wall through a supply pipe 112 having holes 112a in the peripheral wall. The container 111 is rotated to extrude the raw material liquid 114 from the orifices 113 by centrifugal force.
Also, the present inventors have developed and carried out a technique as shown in PTL 4 (see FIG. 11), in which an annular electrode 122 is disposed around a grounded cylindrical container 121 and a high voltage is applied therebetween. This technique makes it possible to induce a larger electrical charge on the container 121, and thus to give a sufficient electrical charge for electrostatic drawing to a raw material liquid jetted from the orifices of the container 121 even if the amount of the jet changes slightly. It therefore becomes possible to produce high quality nanofibers containing no clumps of raw material polymer.
The traveling direction of the raw material liquid extruded radially in the radial direction of the container 121 is deflected by air streams 123 which are substantially perpendicular thereto. Ahead of the deflected raw material liquid is a grounded drum 124. Since the drum 124 is electrically charged due to the application of the high voltage to the annular electrode 122, the raw material liquid or the fibrous material formed therefrom is attracted to the drum 124. A long-strip like collecting member 125 is disposed between the container 121 and the drum 124. The fibrous material attracted to the drum 124 is deposited and collected on the collecting member 125 which is transported in the longitudinal direction.